Popular Science Monthly/Volume 20/January 1882/The Bunsen Lamp
THE great convenience of gas as an illuminating agent, due to its cleanliness and immediate availability in any desired quantity, soon led to its use as fuel; and to-day we have apparatus of all degrees of size and complexity, from the simple burner of the chemical laboratory to the gas-stove with which the meals of a large family may be cooked, or the gas-furnace capable of melting iron or satisfying the demands of the gold and silver assayer, all using gas as fuel—not to speak of the numerous applications of the waste gases from blast-furnaces and the like, or of the Siemens gas-furnace, using gas made especially for it, and in which the degree of heat that can be attained is practically limited only by the capacity of the furnace itself to withstand it.
In all or nearly all the applications of illuminating gas to heating purposes, however, a practical difficulty has to be overcome. The gas is made for illuminating purposes, and therefore burns with a bright flame, which deposits a layer of lamp-black, or soot, on everything brought into contact with it. Evidently this difficulty must be overcome before any extensive use could be made of gas as fuel, particularly for those domestic purposes to which it finds one of its chief applications. For practical purposes one of two methods is adopted. One of these is to cause the gas to burn from a number of small openings in a metal pipe. Two effects are thus produced. The large mass of metal in the tubes abstracts heat from the flame, and, as a result, the latter burns mostly blue and produces little smoke or soot; and, secondly, the distribution of the gas into several small flames allows us to place the article to be heated high enough over the flame to avoid the soot, and yet near enough to get a good heating effect. A second and more common method of preventing the deposition of soot is by the use of some form of the Bunsen lamp.
Nearly all the common gas-stoves and other arrangements for heating by gas are essentially Bunsen lamps, more or less modified to suit particular purposes. One of the simplest and most common forms of this lamp is represented in section in the annexed figure. The gas enters through the metallic tube a, passes through the block b, and finds an outlet through one or more small apertures at c. Surrounding c is a larger metallic tube, d d, having at its base two apertures for the admission of air. . The mixed gas and air rise through the tube d d, and burn at the top with a pale-blue, smokeless flame. The lamp is supported by a heavy cast-iron foot, e.
Though this lamp is of simple construction, the explanation of its operation involves some very curious and interesting facts regarding the theory of flame, as well as some very familiar ones, and in order to comprehend it we shall need to begin with a study of flame in general, and of luminous flames, like that of ordinary gas, in particular.
When a solid combustible, like charcoal, burns in the air, it produces no flame, but simply glows. The blue flame that is often observed playing over the surface of a coal-fire is that of the carbonic-oxide gas produced in the fire, and not that of the coal itself. Only gases burn with a flame. In those cases where a solid or liquid seems to do so, it will he found that it is either volatilized or decomposed by the heat of the combustion, and thus converted into gas before it burns.
Let us now begin our study of the phenomena of flame by considering the flame of hydrogen. Suppose we have hydrogen-gas flowing from a round jet. Just over the opening we have a round column of pure hydrogen. This gas, being lighter than air, and being forced out under some pressure, rises. As it rises it mixes with the air, and we immediately have, surrounding the jet of hydrogen, a layer of mixed hydrogen and air which is inflammable. If we now apply a light this mixture takes fire. The hydrogen unites with the oxygen of the air, forming steam, which is carried away by the current of hot gases; more hydrogen is continually supplied from the jet, and more oxygen from the atmosphere: and thus we have a continuous formation, as fast as it is burned, of this inflammable mixture of hydrogen and oxygen around the central column of hydrogen.
Evidently, then, the flame must be hollow. That it is so may be shown by the familiar experiment of quickly inserting the phosphorus end of a match into the center of the flame, where it may sometimes be held until the wood of the match is burned through without taking fire. The flame can not spread inward, because there is nothing there to support combustion; nor outward, because there is nothing there to burn. The flame is simply that part of the current of gases where the chemical action takes place, and where, consequently, the heat is produced. It is more nearly a place than a thing. If we leave out of account for a moment the chemical changes, we may compare the current of gas which flows from the tube to a metallic rod which is being slowly pushed through a fire. The portion in the fire glows, and, as the rod moves on, different portions of it glow, while the glowing spot, which may be compared to the flame, remains stationary. In the hydrogen-flame the rod is of gas and invisible. We see only the spot which glows, and, as this is stationary, we are apt to regard it as an object by itself, instead of considering it as a spot in a constantly flowing stream.
The hydrogen-flame gives out very little light. If burned from a metallic jet, the flame becomes almost invisible. What little light such a flame does emit, it emits because the gases of which it is composed are hot; but ordinary gases, when heated to the temperature of a flame, emit very little light.
To render a flame luminous in the sense in which an ordinary gas flame is luminous, we must introduce into it some solid which is not converted into gas at the temperature of the flame. The light of a common gas-flame is due to innumerable small particles of carbon, which are separated from the gas in the interior of the flame and are heated white hut while passing through the flame. Illuminating gas consists practically of compounds of carbon and hydrogen. The gas must get very hot before it actually begins to burn, and it is a well known fact that at a high temperature some of the compounds contained in it are decomposed, with separation of carbon.
In the interior of the flame, likewise, the compound of carbon and hydrogen is decomposed, and yields a mixture of carbon and hydrogen. The hydrogen burns with a colorless flame, but the solid particles of carbon, as they float through the flame, are heated white hot, and furnish the light of the flame. Finally, as these glowing particles reach the outside of the flame, where there is more air, they also burn, evolving still more light.
We are now prepared to study the ways in which such a flame may be converted into what is frequently called a "non-luminous" flame, by which is meant, not a flame that gives out no light, for no such flame exists, but one which emits only the faint light of incandescent gases. For brevity I shall use the term "non-luminous" in this article.
A luminous flame may become "non-luminous" from three causes:
2. Dilution of the gas or the air.
3. Too rapid oxidation of the separated carbon.
1. Cooling.—The gas in a flame must be heated to a certain temperature in order that carbon may be separated from it in the manner described.
If the heat of the flame is reduced below this point, no separation of carbon takes place, the flame contains no solid matter, and becomes "non-luminous." For example, if a small gas-flame be caused to play against a cold platinum dish, the flame is spread out over the surface and becomes "non-luminous." The dish, being cold, abstracts so much heat from the flame as to render the separation of carbon impossible. If the dish be heated by a gas-lamp held on the other side, this loss of heat by the flame is checked, and it at once becomes luminous again. A familiar example of the effect of cooling a flame is seen when an ordinary gas-flame is turned very low. Heat is abstracted from the flame by the cooler burner and conducted away, and the result is a small blue flame, emitting scarcely any light. The blue space in the lower part of an ordinary flat gas-flame is likewise due in part to the cooling effect of the burner and in part also to the rush of cold gas into the flame. That this is so is shown by heating the burner, by which means the area of the blue space is notably diminished.
Exact experiments with the photometer have shown that considerably more light is emitted when the gas burns from a red-hot burner, while the consumption of gas is rather diminished than increased. It might seem from these facts that burners of lava, or some similar substance, would have a decided advantage over burners of metal, in regard to the amount of light obtained, since lava is a poor conductor of heat, and would therefore not convey away the heat of the flame so rapidly as metal. Experiment shows, however, that the difference is very slight. It is measurable, but not of practical importance. The practical value of a lava-tip lies simply in the fact that it does not rust. Neither are any arrangements for heating the burner likely to effect any gain in illuminating power. Obviously the heat for this purpose must not be taken from the flame itself, but must either be obtained from its waste heat or from a separate source of heat. The waste heat of the flame would probably not heat the burner hot enough to produce any appreciable effect, while a separate source of heat involves additional expense enough to overbalance any gain likely to accrue.
A flame may be cooled in another way than by means of some cold body held in it, namely, by mixing some indifferent gas with the combustible gas. If, instead of air, a stream of carbonic-acid gas be admitted at the bottom of a Bunsen lamp, the flame becomes "non-luminous." We get the same amount of heat in the flame as before, since we burn the same amount of gas; but this heat has not only to be applied to heating the illuminating gas and the products of its combustion, but has also to heat the carbonic-acid gas. As a consequence, no part of the flame can be so hot as it was before, and the separation of carbon does not take place.
If the tube through which the gases pass be heated, the heat thus added counteracts the cooling effect of the carbonic-acid gas, and the flame becomes luminous again.
2. Dilution.—The second way in which we may render a gas-flame "non-luminous" is by dilution, either of the gas or the air. This can be shown by substituting for the carbonic-acid gas some combustible gas which does not give a luminous flame; for instance, carbonic-oxide gas. This gas can not essentially lower the temperature of the flame, for it burns with as hot a flame as gas, yet the mixture of it with illuminating gas gives a "non-luminous" flame. Plainly, then, we must conclude that simple dilution is able to produce this effect. Apparently such a mixture as this—that is, a mixture of two gases, one of which burns with a luminous and the other with a "non-luminous" flame—requires to be made hotter in order to produce a separation of carbon, and consequently a luminous flame, than does the illuminating gas when burned alone. If we raise the temperature of the flame by heating the burner, it becomes luminous again. Dilution, then, renders a flame "non-luminous," because the temperature of the flame is not high enough to cause a separation of carbon from the diluted gas.
3. Too Rapid Oxidation of the Carbon.—The third method of rendering a flame non-luminous is to supply it with so much oxygen that the carbon is burned at once before it can separate in the solid form.
If we put a small gas-flame into a jar of pure oxygen it is rendered "non-luminous." This effect can not be caused by cooling, for the greater supply of oxygen makes the flame hotter, nor by dilution, for nothing is diluted. Evidently it must be caused by the greater supply of oxygen. The air contains four volumes of nitrogen-gas for every one volume of oxygen. When a substance burns in air, the particles of oxygen are hindered in getting at the combustible by the particles of nitrogen, which are four times as numerous. When the substance burns in pure oxygen there is no trouble of this sort, and the combustion takes place much more rapidly and energetically. Such is the case in our experiment. The oxygen-particles are no longer hindered in getting at the carbon by the inert nitrogen, and therefore seize on it so promptly as to leave none to make the flame luminous. If we allow the carbonic-acid gas, resulting from the burning, to accumulate in the jar, this gas will perform the function of the nitrogen of the air. At first a very bright spot appears in the center of the flame where the least oxygen penetrates. It is brighter than a gas-flame in air, because the more rapid combustion makes the flame hotter, and consequently the little particles of carbon glow more intensely. As the carbonic acid gas accumulates, it becomes more and more difficult for the oxygen to get at the flame. When four fifths of the gas in the jar are carbonic acid, the flame burns as it does in the air. As the carbonic acid still goes on accumulating we get another effect, viz., that of dilution, and the flame becomes "non-luminous" again. In this experiment are illustrated all three of the causes which render luminous gas-flames "non-luminous." At first we observe that the combustion is so rapid that no carbon is separated. At the close of the experiment we see the flame become "non-luminous" again on account of the dilution of the air with inert gas and the attendant cooling of the flame.
If we try this same experiment with a flame which owes its luminosity to incombustible solid particles, or to vapors, we shall get only the effect of the greater heat of the flame. For example, hydrogen-gas containing a small proportion of the vapor of chromyl chloride burns with a luminous flame, the light being due to the separation in the flame of particles of solid oxide of chromium. If this flame be placed in pure oxygen it becomes of an almost dazzling brilliancy. The particles of oxide of chromium are incombustible, and therefore the greater heat of the flame only makes them glow the more brightly. This experiment shows that a flame becomes hotter in pure oxygen than in the diluted oxygen of the air, and furnishes an indirect proof that the reason why a gas-flame is made "non-luminous" in oxygen is because its carbon is burned up so quickly.
We distinguish, then, two effects produced on a luminous gas-flame by pure oxygen. First, it makes the flame hotter, and consequently tends to make it brighter; and, second, if the supply of oxygen is relatively large, it burns up the carbon of an ordinary flame at once, and thus renders the flame "non-luminous." Having now considered the three causes which may convert the luminous flame of our common street-gas into a faint and so-called "non-luminous" flame, we are prepared to trace the operation of these causes in the Bunsen lamp. All the various forms of the Bunsen lamp burn a mixture of gas and air. Now, the air is itself a mixture of four parts of nitrogen with one part of oxygen. The nitrogen of the air takes no part in the process of combustion, but simply passes unchanged through the flame. It is plain that this nitrogen must act on the flame in two ways. First, it must cool it, just as any cold substance passing through the flame would cool it; and, second, the nitrogen dilutes the illuminating gas. The effect of both cooling and dilution, as we have seen, is to make a flame "non-luminous."
But one fifth of the air is oxygen; and this also has two effects upon the flame. It makes it hotter, and it also tends to burn up the carbon of the illuminating gas at once, before it can make the flame luminous. All these causes cooling, dilution and oxidation of the carbon are operative at once in the Bunsen lamp, and the effect that we see is the resultant of all these forces. Probably the most important of them are the cooling and dilution by the nitrogen; for, if the burner through which the gases issue is heated, the flame becomes luminous again.
The Bunsen lamp takes various forms, according to the purpose for which it is to be used. In some of these the gas burns on the top of a piece of fine wire-gauze, after becoming mixed with the air below it. This form of burner is a very common one. It differs in appearance from the usual form of Bunsen lamp, but is essentially the same in principle; that is, it burns a mixture of gas and air, and gives a "nonluminous" flame for the same reasons that the ordinary Bunsen lamp does.